The high cost of energy has led industry to extract usable heat from high temperature waste streams whenever practical. In some applications, a heated waste stream passes over conventional crossflow metallic heat exchanger tubes containing clean ambient air. The ambient air is heated by the waste stream and then typically used as either facility or process heat. In other applications, such as municipal solid waste incineration in which trash and garbage are incinerated to form gaseous products at temperatures up to 2500.degree. F. (1644 K.), water is passed through metallic tubes ("superheater tubes") positioned within the gaseous product stream and converted to steam by the high temperatures. The steam produced by the tube assembly is then used to power a turbine-driven electrical generator.
Although heat extraction from high temperature waste streams using metal-tubed heat exchangers is efficient, two particular problems with metal tubes have been observed. First, the temperature limits of the metals are frequently exceeded by the operating temperatures of the heat exchangers. Second, waste streams are frequently abrasive and/or corrosive and so threaten the physical integrity of the metallic tubes.
To prevent direct attack of the tubes by the products of combustion while allowing the tubes to be superheated, the art has used refractory ceramic shields to cloak the tubes. The refractoriness of these shields provides for high thermal conductivity, integrity at high temperatures, erosion resistance and corrosion resistance. For example, U.S. Pat. No. 4,682,568 discloses a refractory shield comprising a pair of refractory half-shields of identical interchangeable interlocking size and shape, including an interlocking tongue and groove feature ("tongue and groove shields"). See FIG. 1. This design is assembled by applying mortar M to either the superheater tube S or the inner surfaces of the half-shields, attaching one of the half-shields to the outer surface of the superheater tube, positioning the second partial-tube 180 degrees thereto to align the tongues T and the grooves G, and axially engaging the half-shields. This process is repeated until the exterior of each superheater tube is covered. However, this design possesses two drawbacks. First, it requires a clamping mechanism to hold the shields together until the mortar bonds the shields to the tube. Second, the tongue and groove shields may fall off the metallic heat exchanger tubes under extreme service conditions.
FR-A-636 392 discloses a sharp-cornered tongue and groove metallic tube shield combination. During inevitable boiler tube expansion, the metallic boiler tube expands much more than the metallic tube shields, thereby pressing upon the two tube shields and forcing them outward. The design of FR-A-636 392 prevents tube shield movement during boiler tube expansion by insuring that tongue presses against the seat formed by groove. However, the sharp corners of this tongue and groove locking mechanism produce significant stresses, particularly in the corner next to the seat formed by groove. These stresses would be particularly significant in refractory tube shields.
EP-A-0 272 579 discloses a refractory tube shield combination having interlocking tabs and grooves. However, these tabs and grooves also have sharp corners and so suffer from the same stress concentrations discussed above.
Accordingly, there is a need for a refractory tube shield which will not fall from the metallic heat exchanger tube during operation in severe environments.